MOS and other types of transistors are found in many modern semiconductor products where switching and/or amplification functions are needed. Many manufacturing processes and techniques have been developed for forming MOS transistors in semiconductor substrate materials such as silicon and the like. In recent years, the size of transistors and other components have steadily decreased to submicron levels in order to facilitate higher device densities in semiconductor products. At the same time, many applications of such devices have created a need to operate the semiconductor devices at lower power and voltage levels. Accordingly, efforts continue to be made to design semiconductor devices, such as MOS transistors, which consume less power and operate at lower voltages, particularly for logic circuitry.
However, many applications continue to require transistors which operate at higher voltage levels, in addition to those devices designed to operate at low voltages. For example, semiconductor products are often required to interface with equipment such as printers, control systems, or the like, which generate signals at relatively high voltage levels, such as 12 volts DC or higher. In these situations, it is desirable to fabricate transistors designed for low power consumption and low voltage operation, as well as those designed for higher voltages, in a single semiconductor device or product.
For transistors designed for higher power applications, a particular design is often a tradeoff between breakdown voltage and drain-to-source on state resistances (RDSON). Breakdown voltage (BVdss) is often measured as drain-to-source breakdown voltage with the gate and source shorted together. Where high breakdown voltage is needed, drain-extended MOS transistors are often employed, in which one of the source/drain regions is spaced from the gate to provide a drift region or drain extension in the semiconductor material therebetween. The spacing of the drain and the gate spreads out the electric fields thereby increasing the breakdown voltage of the device. However, the drain extension increases the resistance of the drain-to-source current path. In conventional drain-extended MOS devices, the RDSON and breakdown voltage are thus inversely proportional, wherein the drain extension causes an increase in RDSON, thus limiting the drive current rating of the device.
Another problem in MOS transistors is channel hot carrier (CHC) degradation, caused by high electric fields in the channel region of the substrate. High drain currents may ionize electrons and holes through impact ionization, causing injection of hot carriers (electrons and/or holes) into the transistor gate oxide above the silicon substrate. Some of the injected carriers remain in the gate material, which leads to performance degradation and/or device damage, such as a shift in threshold voltage, changed transconductance, and/or changed drain current capability. These channel hot carrier effects thus reduce the operational lifetime of the device. During junction breakdown conditions in conventional drain-extended MOS transistors, the junction breakdown typically occurs near the drain-side end or edge of the gate structure. As a result, CHC degradation is more pronounced at this part of the gate. There remains a need for improved MOS transistor devices and manufacturing techniques for increasing the breakdown voltage and for reducing channel hot carrier degradation, without significant increase in RDSON.